The technique known as the bag technique is very common in the production of mattresses. The bag technique means that springs are wrapped in bags, that is to say, they are individually surrounded by a wrapping material. Thus, springs become flexible in a relatively independent or individual manner in such a manner that they may be flexed individually without affecting nearby springs. Therefore, the comfort of the user is increased and the user's weight is distributed and spread in a more uniform manner over the surface that receives the load.
However, a disadvantage of this type of mattress is that it frequently is very soft for people of great (or different) weight, which complicates mobility in bed as the user turns at several positions throughout sleeping time as the heaviest part of the body plunges more than the rest. This occurs because the caliber of wire is the same in all the springs and those that receive more weight are more compressed; consequently, as time elapses, they wear out and collapse faster thereby reducing the service life of the mattress. Something similar occurs when a person sits repeatedly on the same edge of the mattress. In addition, there is risk of falling from the bed when the user is positioned on the edge of the same or when they sit on the edge, which may cause physical injury.
Patents GB 225,225, U.S. Pat. No. 2,878,012 and U.S. Pat. No. 2,539,003 describe cushions used in vehicle seats with coil springs in which wrappings or coatings are impermeable to air and check valves or the like are provided in order to limit the flow of air in the coating. Consequently, cushioning is provided, which makes coil springs return to a difficult stretched position that reduces oscillation when the vehicle moves on an uneven road or the like. These cushions, however, are not indicated for use in beds.
In addition, U.S. Pat. No. 5,467,489 suggests a coated mattress in which springs are contained in air impermeable capsules and in which check valves are located in the inferior part and in the exit passages of the upper part. This results in flow of air through the mattress, which causes a cooling or refrigerating effect for the user. However, selective cushioning is not obtained. The mattress is also different from the conventional coated mattresses, which consist of separate units that are connected by flexible bonds.
Patent application PCT/NL2005/000226 describes an encapsulated spring unit, adequate for use in mattresses or pillows or the like. This spring unit consists of a helical spring with two axial ends and one closed coating, the coating consisting of one portion of external wrapping stretched between the two axial ends of the spring throughout the external side of the spring, one first portion of the internal coating is stretched inside the spring from one of the ends of the spring, a second portion of internal coating is stretched inside the spring from the opposite end of the spring ends, in which the two internal portions of the coating are joined to each other.